Many systems are known for automatically determining whether an object or person is passing a predetermined location and whether such object or person is authorized to pass such location. If such passage is authorized, the object or person may continue unhindered. If such passage is not authorized, action may be taken for its prevention.
Such systems are often used in theft security systems. A special security tag is placed upon an object, such as an article for sale in a store. If a thief attempts to remove the article from the store, detectors located at the store exits will determine the presence of the tag. Alarms indicate to security personnel that a theft is being committed and the thief may be apprehended. If the article is paid for by a legitimate customer, the tag is removed, deactivated or changed at a check-out station, so that no tag is detected by the detectors at the store exits.
Other applications of such systems may occur in mail or luggage handling systems and personnel and livestock detection or counting systems.
Examples of such applications, systems and associated devices are disclosed in U.S. Pat. Nos. 3,618,059, 3,720,940, 3,754,226, 4,021,705, 4,074,249, 4,095,214, 4,222,517, 4,242,671, 4,274,083, 4,336,531 and 4,342,904.
Some detection systems make use of tags having permanent magnets or radioactive material. Tags with permanent magnets are bulky and heavy. Tags using radioactive material pose health risks. Other systems (e.g., U.S. Pat. No. 4,252,671) may require a piezoelectric tag which retransmits a delayed signal. Yet other systems (e.g., U.S. Pat. Nos. 3,720,940 and 4,021,705) may use a tag having a resonant electronic circuit for retransmitting a generated signal. Some U.S. Pat. Nos. 3,754,226, 4,074,249 and 4,222,517 disclose the use of electromagnetically activated passive signal generator tags. All of the above devices generally can only be used to determine the presence or non-presence of a tag. They cannot determine the specific identity of the tagged object.
It is desirable to have a system not only for detecting objects or persons, but also for automatically identifying same. Such an automatic identification system makes possible a more flexible, less labour-intensive, less expensive mail, baggage or material handling system and more reliable livestock and personnel identification. Other applications for identification systems may be possible.
Examples of such identification systems are disclosed in U.S. Pat. Nos. 3,115,622, 3,618,059, 3,832,530, 4,134,538 and 4,274,083.
Some identification systems (e.g., U.S. Pat. Nos. 3,115,622, 3,618,059 and 4,274,083) disclose the use of an active, battery-operated radio transmitter. Such devices identify a tagged object by detection of a unique radio signal emanating from the transmitter on the tag. However, radio transmitter devices may be expensive. They may also be inconvenient and unreliable because of the limited useful battery life.
The system disclosed in U.S. Pat. No. 3,832,530 depends upon the existence of current pulses in and the absorption of energy within an electronic circuit affixed to a tag. Such current pulses induce voltage pulses in a detector circuit. The current pulses of the tag can be uniquely coded to identify the tagged object. Such a system requires a high degree of sensitivity so as to be able to distinguish small increments of energy absorption in the circuit corresponding to different identification codes.
The system disclosed in U.S. Pat. No. 4,134,538 makes use of electromagnetically activated, passive signal generator tags. Such tags require the selection and dimensioning of magnetic material to provide unique generated signals for each tag to, thus, uniquely identify the tagged object. A disadvantage of such system is that it is difficult to select and size the necessary magnetic material so that the tag will generate a signal capable of being uniquely and reliably distinguishable from the signals of other tags. Such a tag may also be somewhat bulky and cumbersome if the system must have the capacity to handle a large number of objects to be identified.
It is therefore desirable to provide an identification system which is inexpensive, reliable, convenient to use, uses a tag of convenient size, has long life, and has the capacity to handle various numbers of objects that are likely to be required to be handled.